Doppler reactivity augmentation device

ABSTRACT

A fast neutron nuclear reactor contains a nuclear reactor core having an array of device locations. Some device locations in the nuclear reactor core contain fissile and fertile nuclear fuel assembly devices. One or more other device locations in the nuclear reactor core contain Doppler reactivity augmentation devices that amplify the negativity of the Doppler reactivity coefficient within the nuclear reactor core. In some implementations, a Doppler reactivity augmentation device can also reduce the coolant temperature coefficient within the nuclear reactor core. Accordingly, a Doppler reactivity augmentation device contributes to a more stable nuclear reactor core.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is a continuation of U.S. patent applicationSer. No. 14/839,418, filed Aug. 28, 2015, which claims priority to U.S.Provisional Patent Application No. 62/043,210, filed Aug. 28, 2014,which are herein incorporated by reference in their entirety.

BACKGROUND

A fast spectrum nuclear fission reactor (“fast neutron reactors”), suchas a sodium fast reactor, generally includes a reactor vessel containinga reactor core forming an array of device locations for fuel assemblydevices and other reactor support devices. Fissile nuclear fuel issubjected to neutron collisions that result in fission reactions. In abreed-and-burn fast neutron reactor, a fission chain reaction issustained by “fast neutrons” that breed fissile nuclear fuel fromfertile nuclear fuel. Liquid coolant flows through the reactor core,absorbing thermal energy from the nuclear fission reactions that occurin the reactor core. The coolant then passes to a heat exchanger and asteam generator, transferring the thermal energy to steam in order todrive a turbine that generates electricity. Design of such reactorsinvolves combinations of materials, structures, and control systems toachieve desirable operational parameters, including reactor corestability, efficient thermal generation, long-term structural integrity,etc.

SUMMARY

The described technology provides a fast neutron nuclear reactorcontaining a nuclear reactor core having an array of device locations.Some device locations in the nuclear reactor core contain fissile andfertile nuclear fuel assembly devices. One or more other devicelocations in the nuclear reactor core contain Doppler reactivityaugmentation devices that amplify the negativity of the Dopplerreactivity coefficient within the nuclear reactor core. In someimplementations, a Doppler reactivity augmentation device can alsoreduce the coolant temperature coefficient within the nuclear reactorcore. Accordingly, a Doppler reactivity augmentation device contributesto a more stable nuclear reactor core.

In one implementation, the Doppler augmentation device includes vanadiumor a vanadium alloy, such as V-20Ti, V-10Cr-5Ti, V-15Cr-5Ti, V-4Cr-4Ti,V-4Cr-4Ti NIFS Heats 1 & 2, V-4Cr-4Ti US Heats 832665 & 8923864,V-4Cr-4i Heat CEA-J57, etc. In other implementation, other materials andalloys may be employed, including titanium alloys. The vanadium orvanadium alloy (referred to herein as “vanaloys”) may be employed as astructural material (e.g., for pin cladding and assembly ducts).

This Summary is provided to introduce a selection of concepts in asimplified form that are further described below in the DetailedDescription. This Summary is not intended to identify key features oressential features of the claimed subject matter, nor is it intended tobe used to limit the scope of the claimed subject matter.

Other implementations are also described and recited herein.

BRIEF DESCRIPTIONS OF THE DRAWINGS

FIG. 1 illustrates a partial—cutaway perspective view of an examplenuclear fission reactor with a fast nuclear reactor core containing aDoppler reactivity augmentation element.

FIG. 2 illustrates a data chart for the neutron scattering cross-sectionof vanadium (V-51).

FIG. 3 illustrates a data chart for the neutron scattering cross-sectionof titanium (Ti-48).

FIG. 4 illustrates a data chart including the neutron scatteringcross-sections of vanadium and sodium compared with the neutron fluxesof different fast neutron reactors.

FIG. 5 illustrates example reactivity coefficients within a fast nuclearreactor having one or more Doppler reactivity augmentation devicesinserted into device locations of a fast nuclear reactor core.

FIG. 6 illustrates an example cumulative reactivity coefficientinsertion over time, based on an integration of the individualreactivity coefficients as a function of time.

FIG. 7 illustrates a cross-sectional view of an example fast nuclearreactor core having an array of locations of nuclear reactor coredevices, including Doppler reactivity augmentation assembly devices.

FIGS. 8A and 8B illustrate a side plan view and cross-sectional view ofan example Doppler reactivity augmentation device in the form of areactivity control duct.

FIGS. 9A and 9B illustrate a side plan view and cross-sectional view ofan example Doppler reactivity augmentation device in the form of aDoppler augmentation assembly device.

FIG. 10 illustrates a schematic perspective view of a plurality of fuelpins in the form of a Doppler augmentation assembly device.

FIG. 11 illustrates example operations for augmenting Doppler reactivitywithin a nuclear reactor core.

DETAILED DESCRIPTIONS

Fast nuclear reactors are designed to increase the utilizationefficiency of nuclear fuel (e.g., uranium) in fission reactions. Fastreactors can capture significantly more of the energy potentiallyavailable in natural uranium, for example, than typical light-waterreactors. Production of energy in the fast reactor core is intensebecause of the high-energy neutrons that are employed in the fastnuclear reactor core. However, the high burnup and energy intensity infast reactors also stresses the structural material in the nuclear fuelassembly devices to a greater degree relative to light-water reactors.

A particular classification of fast nuclear reactor, referred to as a“breed-and-burn” fast reactor (of which one type is a fast reactor), isa nuclear reactor capable of generating more fissile nuclear fuel thatit consumes. For example, the neutron economy is high enough to breedmore fissile nuclear fuel from fertile nuclear reactor fuel, such asuranium-238 nuclear or thorium-232 fuel, than it burns. The “burning” isreferred to as “burnup” or “fuel utilization” and represents a measureof how much energy is extracted from the nuclear fuel. Higher burnuptypically reduces the amount of nuclear waste remaining after thenuclear fission reaction terminates.

Another particular classification of a fast nuclear reactor is based onthe type of nuclear fuel used in the nuclear fission reaction. A metalfuel fast nuclear reactor employs a metal fuel, which has advantage ofhigh heat conductivity and a faster neutron spectrum than inceramic-fueled fast reactors. Metal fuels can exhibit a high fissileatom density and are normally alloyed, although pure uranium metal hasbeen used in some implementations. In a fast nuclear reactor, minoractinides produced by neutron capture of uranium and plutonium can beused as a metal fuel. A metal actinide fuel is typically an alloy ofzirconium, uranium, plutonium, and minor actinides.

FIG. 1 illustrates a partial—cutaway perspective view of an examplenuclear fission reactor 100 with a fast nuclear reactor core 102containing one or more Doppler reactivity augmentation devices (such asa Doppler reactivity augmentation device 104). Other elements within thefast nuclear reactor core 102 include nuclear fuel assembly devices(such as a nuclear fuel assembly device 106) and movable reactivitycontrol assembly devices (such as a moveable reactivity control assemblydevice 108). Certain structures of the example nuclear fission reactor100 have been omitted, such as coolant circulation loops, coolant pumps,heat exchangers, reactor coolant system, etc., in order to simplify thedrawing. Accordingly, it should be understood that the example nuclearfission reactor 100 may include additional structures not shown in FIG.1.

Implementations of the nuclear fission reactor 100 may be sized for anyapplication, as desired. For example, various implementations of thenuclear fission reactor 100 may be used in low power (−5 Mega Wattthermal) to around 1000 Mega Watt thermal) applications and large power(around 1000 Mega Watt thermal and above) applications, as desired.

Some of the structural components of the fast nuclear reactor core 102may be made of refractory metals, such as tantalum (Ta), tungsten (W),rhenium (Re), or carbon composites, ceramics, or the like. Thesematerials may be selected to address the high temperatures at which thefast nuclear reactor core 102 typically operates. Structuralcharacteristics of these materials, including creep resistances,mechanical workability, corrosion resistance, etc., may also be relevantto selection. Such structural components define an array of devicelocations within the fast nuclear reactor core 102.

The fast nuclear reactor core 102 is disposed in a reactor vessel 110containing a pool of coolant (such as liquid sodium). For example, invarious implementations, a reactor coolant system (now shown) includes apool of liquid sodium disposed in the reactor vessel 110. In such cases,the fast nuclear reactor core 102 is submerged in the pool of liquidsodium coolant in the reactor vessel 110. The reactor vessel 110 issurrounded by a containment vessel 116 that helps prevent loss of theliquid sodium coolant in the unlikely case of a leak from the reactorvessel 110. In alternative implementations, coolant can flow throughcoolant loops throughout the nuclear fission reactor 100.

The fast nuclear reactor core 102 contains the array of device locationsfor receiving various reactor core devices, such as nuclear fuelassembly devices, reactivity control assembly devices and Dopplerreactivity augmentation devices, within the central core region 112. Anin-vessel handling system (not shown) is positioned near the top of thereactor vessel 110, at about location 114, and is configured to shuffleindividual reactor core devices in and/or out of the device locationswithin the fast nuclear reactor core 102. Some reactor core devices maybe removable from the fast nuclear reactor core 102, while other reactorcore devices may not be removable from the fast nuclear reactor core102.

The fast nuclear reactor core 102 can include a nuclear fission igniterand a larger nuclear fission deflagration burn-wave-propagating region.The nuclear fission igniter provides thermal neutrons for the fissionreaction of fissile nuclear fuel. The larger nuclear fissiondeflagration burn-wave-propagating region may contain thorium (Th) oruranium (U) fuel and functions on the general principles of fast neutronspectrum fission breeding.

In one implementation, the nuclear fuel within a nuclear fuel assemblydevice may be contained within fissile nuclear fuel assembly devices orfertile nuclear fuel assembly devices. The difference between fissilenuclear fuel assembly devices or fertile nuclear fuel assembly devicesis effectively the enrichment level of the nuclear fuel, which canchange over time within the fast nuclear reactor core 102. Structurally,fissile nuclear fuel assembly devices or fertile nuclear fuel assemblydevices can be identical in some implementations. The nuclear fuelassembly device 106 in the fast nuclear reactor core 102 can include asolid hexagonal tube surrounding a plurality of fuel elements, such asfuel pins, which are organized into the nuclear fuel assembly device106. Non-hexagonal tubes may also be used on some implementations. Thetubes in a nuclear fuel assembly device 106 allow coolant to flow pastthe fuel pins through interstitial gaps between adjacent tube walls.Each tube also allows individual assembly orificing, provides structuralsupport for the fuel bundle, and transmits handling loads from ahandling socket to an inlet nozzle. Fuel pins typically consist ofmultiple nuclear fuel rods (such as uranium, plutonium or thorium)surrounded by a liner and cladding (and sometimes an additionalbarrier), which prevents radiative material from entering the coolantstream. Individual pins of a nuclear fuel assembly device 106 in thefast nuclear reactor core 102 can contain fissile nuclear fuel orfertile nuclear fuel depending on the original nuclear fuel rod materialinserted into the pin and the state of breeding within the pin.

The moveable reactivity control assembly device 108 can be inserted intoand/or removed from the central core region 112 by the in-vesselhandling system to provide real-time control of the fission process,balancing the needs of keeping the fission chain reaction active whilepreventing the fission chain reaction from accelerating beyond control.The state of a fission chain reaction is represented by an effectivemultiplication factor, k, which indicates the total number of fissionevents during successive cycles of the chain reaction. When a reactor isin a steady state (i.e., each individual fission event triggers exactlyone subsequent fission event), k equals 1. If k>1, the reactor issupercritical and the reaction rate will accelerate. If k<1, the reactoris subcritical and the fission rate will decrease. Conditions within thecentral core region 112 change over time. Hence, moveable reactivitycontrol assemblies may be used to adjust the multiplication factor ofthe fission chain reaction as conditions change.

The moveable reactivity control assembly device 108 is a highlyeffective neutron absorbing mechanical structure that can be activelyinserted into or removed from the central core region 112 while thefission process is occurring. A moveable reactivity control assemblydevice includes chemical elements of a sufficiently high neutron capturecross-section to absorb neutrons in the energy range of the nuclearfission reaction, as measured by its absorption cross-section. As such,the moveable reactivity control assembly device 108 influences thenumber of neutrons available to cause a fission reaction within the fastnuclear reactor core 102, thereby controlling the fission rate of thefissile nuclear fuel within the fast nuclear reactor core 102. Examplematerials used in moveable reactivity control assembly devices of thefast nuclear reactor 100 include without limitation boron carbide, analloy of silver, indium, and cadmium, or a hafnium-hydride. Bycontrolling the portion of the moveable reactivity control assemblydevice 108 (as well as the number of moveable reactivity controlassemblies) that interacts with the fission reaction within the centralcore region 112, the multiplication factor can be tuned to maintainreactor criticality. Accordingly, a moveable reactivity control assemblydevice 108 represents an adjustable parameter for controlling thenuclear fission reaction.

The Doppler reactivity augmentation device 104 contains one or morematerials capable of altering the Doppler reactivity coefficient withina nuclear fission reaction of the fast nuclear reaction core 102. Forexample, the Doppler reactivity augmentation device 104 can amplify thenegativity of the Doppler reactivity coefficient within the fast nuclearreactor core 102. In some implementations, a Doppler reactivityaugmentation device 104 can also reduce the coolant temperaturecoefficient within the fast nuclear reactor core 102. The Dopplerreactivity coefficient may be considered the fuel temperaturecoefficient of reactivity, representing the change in reactivity perdegree of change in the temperature of the nuclear fuel. The Dopplerreactivity coefficient arises from or is caused by the Dopplerbroadening effect, which refers to the broadening of spectral linescaused by distribution of relative velocities of neutrons and fuelnuclides within the fast nuclear reactor core 102. The Dopplerreactivity coefficient is modified by changing the spectrum of thenuclear fission reactor 100. Faster neutrons are scattered to lowerenergies at levels where the Doppler broadening effect can occur. Nearlyall Doppler broadening occurs below 10 keV. For example, captureresonances for uranium-238-based metal fuel in the 0.8-3 keV energyrange exhibit substantial Doppler broadening as the reaction environmenttemperatures rise. Accordingly, increasing the neutron flux within theDoppler broadening energy range enhances the transmutation of fertilenuclear fuel into fissile nuclear fuel.

In one implementation, the nuclear fission reactor 100 is a fastspectrum nuclear fission reactor having an average neutron energy ofgreater than or equal to 0.1 MeV.

In one implementation, one or more materials in the Doppler reactivityaugmentation device 104 introduce a large positive contribution byenhancing elastic scattering for neutrons having an energy within theDoppler broadening energy range of the fertile nuclear fuel of the fastnuclear reaction core 102. In this manner, neutrons having an energy inthe primary Doppler broadening energy range of the nuclear fuel areelastically scattered within the central core region 112, increasing theprobability of neutrons colliding with fertile fuel nuclides andresulting in enhanced transmutation reactions to breed fissile nuclearfuel. Thus, for uranium-238-based metal fuel, the greater number ofneutrons scattered in the 0.8-3 keV energy range, the greatercontribution made by Doppler broadening to the probability of atransmutation or breeding reaction.

Vanadium is an example element that is characterized by a neutronscattering cross-section that overlaps the Doppler broadening energyrange of uranium-238-based fuels. Several vanadium alloys (“vanaloys”)are also characterized by a neutron scattering cross-section thatoverlaps the Doppler broadening energy range of uranium-238-based fuels,including without limitation V-20Ti, V-10Cr-5Ti, V-15Cr-5Ti, V-4Cr-4Ti,V-4Cr-4Ti NIFS Heats 1 & 2, V-4Cr-4Ti US Heats 832665 & 8923864, andV-4Cr-4i Heat CEA-J57. Titanium and titanium alloys have been found toprovide a similar although less appreciable effect. Alloys having atleast 10% vanadium or titanium by mass can provide a substantial Dopplerreactivity augmentation benefit by increasing the negative Dopplerreactivity coefficient feedback, although alloys having 30% or morevanadium or titanium generally produce better results, as dovanadium-based or titanium-based alloys having 50% or greater vanadiumor titanium by mass.

As shown in the data chart 200 in FIG. 2, vanadium is characterized by aneutron scattering cross-section 202 having three large resonances (at204) and a tail 206 near or below 10 keV. A region 208 shows the primaryDoppler broadening region of uranium-238-based fuels. Note theoverlapping of at least one resonance and the associated tail 206 withthe primary Doppler broadening region 208. Titanium and titanium alloysare another example of materials that can enhance neutron scatteringwithin the Doppler broadening energy range of uranium-238-based fuels.As shown in the data chart 300 in FIG. 3, titanium is characterized by aneutron scattering cross-section 302 having a large resonance 304 nearjust above 10 keV and a large tail 306 below 10 keV. A region 308 showsthe primary Doppler broadening region of uranium-238-based fuels. Notethe overlapping tail 306 with the primary Doppler broadening region 308.Accordingly, vanadium, titanium, and certain of their alloys are examplematerials for use in the Doppler reactivity augmentation device.

FIG. 4 illustrates a data chart 400 including the neutron scatteringcross-sections of vanadium (402) and sodium (404) compared with theneutron fluxes of different fast neutron reactors. The neutron flux of afast reactor employing HT9 stainless steel is shown in FAST REACTOR-Sdata 408, and the neutron flux of a fast reactor in which some or all ofthe HT9 stainless steel of the FAST REACTOR-S reactor core is replacedwith vanadium or a vanadium alloy is shown in FAST REACTOR-V data 406.Neutron flux refers to the total length travelled by all neutrons perunit time and volume or, nearly equivalently, the number of neutronstravelling through a unit area in unit time. In either case, a greaterneutron flux yields a greater probability of a neutron colliding with anuclear fuel atom.

The four main reactivity coefficients in a fast neutron sodium-coolednuclear reactor are, listed in chronological order: Doppler, axial,sodium, and radial. Reactivity coefficients parametrize the change inreactivity per degree of change in the temperature of the nuclear fuelresulting from various contributors. As previously described, theDoppler reactivity coefficient parametrizes the change in reactivity perdegree of change in the temperature of the nuclear fuel resulting fromDoppler broadening. The axial reactivity coefficient parametrizes thechange in reactivity per degree of change in the temperature of thenuclear fuel cladding, which causes core axial fuel expansion. Thesodium reactivity coefficient parametrizes the change in reactivity perdegree of change in the temperature of the coolant, which causesexpansion/voiding. (A coolant reactivity coefficient represents a moregeneral version of the sodium reactivity coefficient.) The radialreactivity coefficient parametrizes the change in reactivity per degreeof change in the temperature of the assembly duct, which causes coreradial fuel expansion. A negative net/total reactivity coefficientprovides a negative feedback on reactivity as the temperature of thenuclear fuel increases, thereby contributing to the stabilization of thenuclear fission reactor—as fuel temperature increases, reactivitydecreases—contributing to a self-stabilizing nuclear fission reaction.

FIG. 5 illustrates example reactivity coefficients 500 within a fastnuclear reactor having one or more Doppler reactivity augmentationdevices inserted into device locations of a fast nuclear reactor core.The reactivity coefficients are listed in chronological order from leftto right. The Doppler reactivity coefficient is increased in the fastnuclear reactor core having one or more Doppler reactivity augmentationdevices as compared to a fast nuclear reactor core having no Dopplerreactivity augmentation devices. Furthermore, the sodium reactivitycoefficient is decreased in the fast nuclear reactor core having one ormore Doppler reactivity augmentation devices as compared to a fastnuclear reactor core having no Doppler reactivity augmentation devices.However, as previously described, each reactivity coefficientcontributes to the reaction in a chronological manner. As such, thecumulative reactivity coefficient “insertion” at each point in time isconsidered in FIG. 6.

Table 1 shows example data showing changes in the four main reactivitycoefficients in a fast neutron sodium-cooled nuclear reactor between aFAST REACTOR-S and a FAST REACTOR-V.

TABLE 1 Reactivity Coefficients in a FAST REACTOR-S and a FAST REACTOR-VFast Fast Reactor with Coefficient Reactor Vanadium (cents/K) with SteelAlloy Change(Δ) Change(%) Doppler −0.0985 −0.151 −0.053   53% Axial−0.0939 −0.0687 0.025 −27% Sodium 0.216 0.167 −0.049 −23% Radial −0.174−0139 0.035 −20% Net −0.150 −0.191 −0.041   27%

FIG. 6 illustrates an example cumulative reactivity coefficientinsertion 600 (“net reactivity insertion”) over time, based on anintegration of the individual reactivity coefficients as a function oftime. Because the combination of the Doppler reactivity coefficient isincreased and the sodium reactivity coefficient is decreased in the fastnuclear reactor core having one or more Doppler reactivity augmentationdevices, the net reactivity insertion remains negative at each point intime, providing negative reactivity feedback as fuel temperature risesand contributing to enhanced stability of the fission reaction.

FIG. 7 illustrates a cross-sectional view 700 of an example fast nuclearreactor core 702 having an array of locations (such as device location704) of nuclear reactor core devices, including Doppler reactivityaugmentation devices. It should be understood that a fast nuclearreactor core typically has more device locations and devices than shownin the example core of FIG. 7, but a reduced number of device locationsand devices is shown to facilitate description and illustration. Eachdevice is inserted into a structurally-defined device location withinthe array. Reflector devices, such as a replaceable radiation reflectordevice at the device location 704, and permanent radiation reflectormaterial 714 are positioned at the boundary of the central reactor coreregion to reflect neutrons back into the central reactor core region.

FIGS. 8A and 8B illustrate a side plan view and cross-sectional view ofan example Doppler reactivity augmentation device 800 in the form of areactivity control duct. In one implementation, the outer structuralwall 804 of the reactivity control duct form of the Doppler reactivityaugmentation device 800 is formed from a Doppler reactivity augmentationmaterial having a neutron scattering cross-section resonance peak near10 keV, and therefore, contributing to enhanced elastic neutronscattering in the Doppler broadening energy range of the nuclear fuel(e.g., of the fertile nuclear fuel uranium-238). Example Dopplerreactivity augmentation materials include without limitation vanadium,vanadium alloys, titanium, and titanium alloys. The outer structuralwall 804 forms a channel allowing for the flow of a liquid coolant, suchas liquid sodium. The Doppler reactivity augmentation device 800 may bemoved in and out of (e.g., inserted into or removed from) devicelocations of the nuclear reactor core, although the Doppler reactivityaugmentation device 800 may also be fixed in the nuclear reactor core.

FIGS. 9A and 9B illustrate a side plan view and cross-sectional view ofan example Doppler reactivity augmentation device 900 in the form of aDoppler augmentation assembly device. In one implementation, the outerstructural wall 904 of the reactivity control duct form of the Dopplerreactivity augmentation device 900 is formed from stainless steel (e.g.,HT9) and encompasses a core 906 of Doppler reactivity augmentationmaterial having a neutron scattering cross-section resonance peak near10 keV, and therefore, contributing to enhanced elastic neutronscattering in the Doppler broadening energy range of the nuclear fuel(e.g., of the fertile nuclear fuel uranium-238). Example Dopplerreactivity augmentation materials include without limitation vanadium,vanadium alloys, titanium, and titanium alloys. In one implementation,the core 906 includes one or more channels (such as a channel 908)allowing for the flow of a liquid coolant, such as liquid sodium. TheDoppler reactivity augmentation device 900 may be moved in and out of(e.g., inserted into or removed from) device locations of the nuclearreactor core, although the Doppler reactivity augmentation device 800may also be fixed in the nuclear reactor core.

As described above in respect to FIG. 1, a Doppler reactivityaugmentation device may be a nuclear fuel assembly device. FIG. 10 showsa nuclear fuel assembly device 1000 as a Doppler reactivity augmentationdevice. The nuclear fuel assembly device 1000, which can be in thenuclear reactor core (e.g., core 102 of FIG. 1) can include a solidhexagonal tube 1012 surrounding a plurality of fuel elements, such asfuel pins 1010, which are organized into the nuclear fuel assemblydevice 1000. Non-hexagonal tubes may also be used on someimplementations. The tubes 1012 in a nuclear fuel assembly device 1000allow coolant to flow past the fuel pins 1010 through interstitial gapsbetween adjacent tube walls 1012. Fuel pins 1010 typically consist ofmultiple nuclear fuel rods 1014 (such as uranium, plutonium or thorium)surrounded by a liner 1016 and/or a cladding 1018 (and/or sometimes anadditional barrier 1020), which prevents radiative material fromentering the coolant stream. Individual pins 1010 of a nuclear fuelassembly device 1000 in the fast nuclear reactor core can containfissile nuclear fuel or fertile nuclear fuel. There may be one or moregrid spacers 1022 separating the nuclear fuel pins 1010. The Dopplerscattering material may be in one or more of the liner 1016, thecladding 1018, the barrier 1020, or the grid spacer(s) 1022.

FIG. 11 illustrates example operations 1100 for augmenting Dopplerreactivity within a nuclear reactor core. A construction operation 1102constructs a nuclear fission reactor core having an array of defineddevice locations, such as individual device locations formed fromrefractory materials. In one implementation, the nuclear fission reactorcore resides in a liquid metal fueled, liquid sodium-cooled fast neutronbreed-and-burn fission reactor system. Another construction operation1104 constructs a Doppler reactivity augmentation device that contain aDoppler augmentation material as a structural component of the device,although Doppler reactivity augmentation devices may additionally oralternatively include a Doppler augmentation material as anon-structural material.

An insertion operation 1106 inserts the Doppler reactivity augmentationdevice into a device location of the nuclear fission reactor core. Anoperating operation 1108 operates the fast reactor to cause elasticscattering of neutrons from the Doppler reactivity augmentation devicewithin the broadening range of fertile nuclear fuel of the nuclearfission core of the fast reactor. A removal operation 1110 removes theDoppler reactivity augmentation device from a device location of thenuclear fission reactor core (e.g., for maintenance). Accordingly, insome implementations, a Doppler reactivity augmentation device can bemoveable, while in other implementation a Doppler reactivityaugmentation device may be fixed within the reactor core.

The above specification, examples, and data provide a completedescription of the structure and use of exemplary embodiments of theinvention. Since many implementations of the invention can be madewithout departing from the spirit and scope of the invention, theinvention resides in the claims hereinafter appended. Furthermore,structural features of the different embodiments may be combined in yetanother implementation without departing from the recited claims.

1.-41. (canceled)
 42. A fast nuclear fission reactor comprising: a fast neutron nuclear reactor core containing a central core array structure having multiple device locations; a nuclear fuel assembly inserted in a first device location of the fast neutron nuclear reactor core and containing nuclear fuel characterized by Doppler broadening constrained within a defined energy range; and a Doppler reactivity augmentation device, shaped to be interchangeable with the nuclear fuel assembly, inserted in a second device location of the fast neutron nuclear reactor core and formed at least in part from a scattering material characterized by a neutron scattering cross-section having at least one resonance within the defined energy range of the nuclear fuel of the nuclear fuel assembly to provide down scattering of fast neutrons.
 43. The fast nuclear fission reactor of claim 42, wherein the scattering material includes vanadium or a vanadium-based alloy.
 44. The fast nuclear fission reactor of claim 42, wherein the scattering material includes titanium or a titanium-based alloy.
 45. The fast nuclear fission reactor of claim 42, wherein the defined energy range defines an energy range between 1 keV and 30 keV inclusively.
 46. The fast nuclear fission reactor of claim 42, wherein the Doppler reactivity augmentation device includes a structural form of the scattering material.
 47. The fast nuclear fission reactor of claim 42, wherein the Doppler reactivity augmentation device includes a control duct having an external structural wall formed at least in part from the scattering material.
 48. The fast nuclear fission reactor of claim 42, wherein the Doppler reactivity augmentation device includes an assembly having an external structural wall encompassing a core of the scattering material.
 49. The fast nuclear fission reactor of claim 48, wherein the core of scattering material includes one or more channels configured to allow liquid coolant to flow through the Doppler reactivity augmentation device.
 50. The fast nuclear fission reactor of claim 42, wherein the fast nuclear fission reactor is configured to operate with an average neutron energy of greater than or equal to 0.1 MeV.
 51. The fast nuclear fission reactor of claim 42, wherein a net reactivity insertion profile, based on a chronological order of a Doppler reactivity coefficient, an axial reactivity coefficient, a coolant reactivity coefficient, and a radial reactivity coefficient, remains negative during a fission reaction within the fast nuclear fission reactor.
 52. The fast nuclear fission reactor of claim 42, wherein the Doppler reactivity augmentation device includes a reactivity control assembly device containing a structural form of the scattering material.
 53. The fast nuclear fission reactor of claim 42, wherein the Doppler reactivity augmentation device includes cladding containing a structural form of the scattering material.
 54. The fast nuclear fission reactor of claim 42, wherein the Doppler reactivity augmentation device includes wire wrap formed of the scattering material.
 55. A method of operating a nuclear fission reactor comprising: inserting a nuclear fuel assembly in a first device location of a fast neutron nuclear reactor core, the nuclear fuel assembly containing nuclear fuel characterized by Doppler broadening constrained within a defined energy range; and inserting a Doppler reactivity augmentation device in a second device location of the fast neutron nuclear reactor core, the Doppler reactivity augmentation device being shaped to be interchangeable with a nuclear fuel assembly and formed at least in part from a scattering material to provide down scattering of fast neutrons, the scattering material being formed from an alloy having at least 10% vanadium or titanium by mass.
 56. The method of claim 55, wherein the Doppler reactivity augmentation device includes a control duct having an external structural wall formed at least in part from the vanadium or the vanadium alloy.
 57. The method of claim 55, wherein the Doppler reactivity augmentation device includes an assembly having an external structural wall encompassing a core of the vanadium or the vanadium alloy.
 58. The method of claim 57, wherein the core of the vanadium or the vanadium alloy includes one or more channels configured to allow liquid coolant to flow through the Doppler reactivity augmentation device.
 59. The method of claim 58, further comprising: flowing liquid coolant through the one or more channels of the Doppler reactivity augmentation device.
 60. The method of claim 55, further comprising: operating the nuclear fission reactor with an average neutron energy of greater than or equal to 0.1 MeV.
 61. The method of claim 55, wherein a net reactivity insertion profile, based on a chronological order of a Doppler reactivity coefficient, an axial reactivity coefficient, a coolant reactivity coefficient, and a radial reactivity coefficient, remains negative during a fission reaction within the fast nuclear fission reactor. 